The in-house tiff at the top of the GOP escalated on Wednesday when Trump used a 90-minute sit-in with members of the press and his cabinet to fire back at the former presidential candidate.

In the opinion column published Tuesday, Romney wrote: "It is well known that Donald Trump was not my choice for the Republican presidential nomination. After he became the nominee, I hoped his campaign would refrain from resentment and name-calling."

"It did not," he said.

In a nutshell, Trump's behavior and particularly his actions in December are proof "that the president has not risen to the mantle of the office," Romney's op-ed reads.